OCT imaging yields better visualization

Reviewed by James Fujimoto, PhD

Future increases in imaging speed will improve performance and functionality for OCT and OCT angiography (OCTA), said James Fujimoto, PhD.

One key advancement is the extension of structural OCT to perform functional imaging, said Dr. Fujimoto, the Elihu Thomson Professor of Electrical Engineering and Computer Science, Research Laboratory of Electronics and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA.

He focused on OCTA, noting some of its advantages, such as visualization of the microvasculature by motion contrast from moving blood without requiring dye injection, depth-resolved images of retinal and choriocapillaris vasculature, and high-contrast images that are not obscured by dye leakage.

At the same time, OCTA is prone to artifacts and requires considerable experience to interpret. From a technical viewpoint, OCTA requires longer imaging times and has a more limited retinal coverage than structural OCT. It also has a limited dynamic range, making it difficult to detect flow impairment, he noted.

Ultra-high-speed, swept-source OCT and OCTA help to meet some of these challenges, Dr. Fujimoto said.

Research instruments can operate at 400 kHz A-scan rates-four times faster than commercial instruments, he noted.

“It’s also possible to use eye tracking and software motion correction to increase the field of view and to average multiple volumes, improving image quality,” Dr. Fujimoto said. “The question is: Will it be possible to measure differences in flow?”

Higher speeds will enable OCTA with more and faster repeated B-scans and variable interscan time analysis (also called VISTA) to differentiate capillary flow impairment (Choi, Moult et al. Ophthalmology. 2015; Ploner S, et al. Retina. 2016). Dr. Fujimoto also pointed to several examples in patients with conditions including diabetic retinopathy, age-related macular degeneration, and geographic atrophy.

En face Doppler OCT

En face Doppler OCT

Dr. Fujimoto highlighted another functional extension of OCT, called en face Doppler OCT, which enables quantitative measurements of total retinal blood flow.

In a preliminary study using ultra-high-speed, volumetric Doppler OCT, total retinal blood flow was measured in 41 eyes from 31 patients-including 20 eyes without diabetic retinopathy from 11 patients with diabetes and 16 healthy eyes from 12 healthy subjects. The study found a consistent decrease in blood flow in patients with diabetic macular edema (Lee B, et al. JAMA Ophthalmology. 2017).

Dr. Fujimoto believes that increases in OCT imaging speed can improve OCTA and Doppler OCT quality and enable new functional imaging techniques.

“Changes in vasculature structure and flow impairment are early markers for many diseases,” he said. “Automated analysis methods will also be very important because OCTA and Doppler OCT data are very complex.”

On one hand, there are powerful advances in technology, but the challenge is their high cost. These new technologies are important research tools, although larger clinical studies are needed to determine if they will be useful in routine clinical practice.

“Generally speaking,” Dr. Fujimoto concluded, “this is a very exciting time for ophthalmic imaging.”

James Fujimoto, PhD

E: jgfuji@mit.edu

This article was adapted from Dr. Fujimoto’s presentation at the 2016 meeting of the American Academy of Ophthalmology. Dr. Fujimoto has equity in Optovue and receives royalties from IP owned by MIT and licensed to Carl Zeiss Meditec and Optovue.